The purpose of this project is to elucidate the interactions between proteins of plasma and platelet origins which seem to be essential for the formation of a hemostatic plug. Protein-protein interactions will be studied between contractile platelet components and fibrin and the possible role of the transamidating enzyme, activated factor XIII, will be examined. Two categories of methods will be used: chemical methods for the assessment of the transamidase-mediated cross-linking processes and physical methods for studying the effect of activated factor XIII on the isometric contractile and viscoelastic properties of platelet protein-fibrin hybrid systems.